Author Bio

Mark Thompson has been in the industry for 29 years. He started out in Stockton, California in 1980 as a driller and photo tech right out of high school. He moved to Santa Clara soon thereafter, and worked for Multiplex (which became ACI West). In 1993 he relocated to the Pacific Northwest, where he worked for Pacific Circuits, (which became TTM) and Praegitzer Industries. He started working with Prototron in 1996, and has been with the company ever since.

In his free time, Mark is an avid pilot, with a fondness for the aircraft of yesteryear. Mark says, "I have been privileged to fly many biplanes from the 1930s and 1940s, including Fleets, Stearmans and N3Ns."

The Bare (Board) Truth: Getting on the Same Page—A Data Story

In this month’s column, I will discuss what makes a great incoming dataset for PCB fabrication.

Drawings and README Files

The overall part thickness should be expressed as the desired thickness measured between two points, including tolerances. For example:

0.062” ±10% metal-to-metal including surface finish

This description tells us the part will be 0.062” nominal with an allowed variance of ±10% measured over the finished plated metal and surface finish. If the drawing or README file merely states 0.062”, the fabricator must establish whether or not this is a maximum thickness or if a tolerance is associated with it. The note also says nothing about what distance 0.062” refers to. Is it 0.062” glass-to-glass over the material, or is it metal-to-metal? Does this include plate-up and surface finish, or is it an overall dielectric with a tolerance, such as ±10%?

Thickness callouts for single-sided or double-sided orders are even more critical. As a fabricator, we can control the thickness of the multilayer by using different combinations of prepregs/cores. If a customer calls out a single-sided or double-sided job as 0.008”, is this the core dielectric or an overall dielectric? If 0.008” represents the core dielectric callout on a 2-ounce finished part, the final thickness would be closer to 0.013”. If the callout for 0.008” pertains to the overall finished thickness, we would need to start at a 0.004” core to finish at approximately 0.009” after plate, surface finish, and mask. Again, notes about thickness should ideally describe what the overall thickness is to be and the points from which they should be measured.

Another example of drawing callouts that can require clarification is copper callouts. If the drawing or README file expresses the copper merely as 2-ounce copper, this leaves the fabricator with more questions. Is it a starting or finished copper callout? If the part is a multilayer board, is it 2-ounce inners and 2-ounce finished outers? If that’s the case, a fabricator will start on 1-ounce copper clad outers and plate to a 1-ounce finish. IPC dictates that there be a minimum of 0.0008” copper in the barrels of the holes for conductivity concerns. Fabricators typically plate up in whole ounce increments to ensure this minimum requirement is met, so a part that started on half-ounce copper foil would finish at 1.5 ounces after plate.

If a note merely reads 1-ounce finish, the fabricator must call the customer to clarify their intentions because starting on 0.05-ounce foil and plating to 1 ounce does not meet IPC minimum requirements for hole wall thickness.

To read this entire column, which appeared in the October 2018 issue of Design007 Magazine, click here.

2018

Thickness callouts for single-sided or double-sided orders are even more critical. As a fabricator, we can control the thickness of the multilayer by using different combinations of prepregs/cores. If a customer calls out a single-sided or double-sided job as 0.008”, is this the core dielectric or an overall dielectric? If 0.008” represents the core dielectric callout on a 2-ounce finished part, the final thickness would be closer to 0.013”. If the callout for 0.008” pertains to the overall finished thickness, we would need to start at a 0.004” core to finish at approximately 0.009” after plate, surface finish, and mask.

One of the biggest issues PCB fabricators face is the completeness (or incompleteness) of the data output package we receive from customers on a new PCB. In this column, I am going to present what is needed, from a fabricator’s perspective, for a good output package and why.

2016

I get this phone call once a week: “Mark, what is the smallest mechanical via that can be done by your company?” I reply, “What will the tolerance for the vias in question be?” If they say, “Oh, your standard +/-.003” tolerances,” I must tell them the min via would be around .0078” with a signal pad of at least .014” and an anti-pad of at least .018”. What if they don't have that kind of room?

Can you truly increase profitability through PCB design practices? Mark Thompson believes you can. And it starts with a philosophy that embraces DFM techniques. Then you must be ready for the initial release to a fabricator by ensuring that you are communicating all of your specifications and needs clearly to the fabrication house so that you get an accurate quote. Let’s dive in, starting with Number 10 and working our way to the most important way a designer can increase company profits.

2015

In this column, we will once again be focusing on controlled impedance structures, both from the layout side and the simulation side. I will break them down into the sub-categories of the models they represent and the important points to remember when using the various models. I will also be asking questions such as, “Why would a fabricator ask for a larger impedance tolerance?” and “Where does the fabricator draw the line for controlling various structures?”

This column is about meeting each customer's needs. Some customers' requirements are as simple as a specific definition for a fiducial size, rail tooling, or orientation feature, while other customers may require special processes. Mark Thompson offers fabricator tips that can help designers get the boards they need.

I’ve been thinking over what 2015 might look like, from my point of view at a PCB fabrication company. Let me first start out with some broad overviews of trends from 2014 that I see continuing. On my end, I certainly expect to see more RF work, more hybrid analog-digital PCBs, and more surface finishes for lead-free assemblies. And that’s just the tip of the iceberg.

2014

Not all board fabricators have the ability to have both CAD and CAM. You may say to yourself, "But a CAM tool should be able to do some, if not all, CAD functions," and that is true; but if you are really getting to the design level, you need to have a design team.

Not all board fabricators have the ability to have both CAD and CAM. You may say to yourself, "But a CAM tool should be able to do some, if not all, CAD functions," and that is true; but if you are really getting to the design level, you need to have a design team.

2013

Columnist Mark Thompson writes, "Based on today's board complexities, a review should be done prior to quote to make sure no manufacturing issues occur. This is critical when it comes to things like minimum pre-preg interfaces on high-copper coil boards or jobs with unique reference planes for various impedance scenarios."

Columnist Mark Thompson writes, "Based on today's board complexities, a review should be done prior to quote to make sure no manufacturing issues occur. This is critical when it comes to things like minimum pre-preg interfaces on high-copper coil boards or jobs with unique reference planes for various impedance scenarios."

In this column, Mark Thompson revisits topics covered in some of his previous columns and fleshes them out with new, updated information. Thompson says, "In this job, I truly learn something every day, and I'm happy to share a few notable nuggets with you."

2012

In this column, Mark Thompson shows that fabricators are not necessarily meddling in your design; some of them really do want to help make your board right the first time. And he also demonstrates how patience and perseverance can go a long way with a customer!

Designers continue to create the same-net spacing violations when relying on autorouters. Surface features connected elsewhere on an internal plane may have copper pour too close to other metal features. Electrically it doesn't matter whether these features bridge, but for most fabricators, any sliver thinner than 0.003" has the potential to flake off and redeposit elsewhere. By Mark Thompson.

A very large customer sent us two 4-layer boards riddled with differential pairs, with no information about any controlled impedances or specific dielectrics. When we asked if these were to be controlled, the customer was most appreciative and realized that some mention of the impedances, threshold and tolerance should have been made initially. When in doubt, talk to the customer!

It was a mostly sunny week in San Diego, where IPC APEX EXPO returned after a long absence. I thought the San Diego Convention Center was a great choice for a venue. And this year, the engineers and designers on the show floor were looking at new processes and technologies like kids in a candy store.

2011

You may be tempted to cut corners in an effort to stay on schedule. But cutting corners to save time does not save anything if it results in a new rev. Let's talk about the risks associated with assuming your board house will find and be able to correct errors in your designs. You'll avoid most of these if you slow down and do it right!

A few months ago, I covered the "10 Most Common Fab Misconceptions." In this column, I will take a similar approach and address four of the most common fabrication questions that I hear. These same questions keep popping up, over and over. But I believe I can dispel the myths surrounding these challenges, and explain their solutions.

I'm always amazed at the diversity of people I see while people-watching in Vegas. And this year, we saw a great diversity of new products and processes at APEX. Some were new combinations of older technologies, while others addressed problems in a completely new, different way.

2010

Let's start by clarifying the intent of the netlist compare. I still get requests to just "generate a netlist" based on the customer's Gerbers. As I have said, since the intent of a netlist compare is to compare the design criteria against the exported Gerber files, this would never find a mismatch.

Most of you have heard of the European Union's RoHS directive. Some people mistakenly think it's mainly an assembly problem. But how, exactly, does RoHS pertain to PCB fabricators and designers? Is RoHS-compliant the same as RoHS-compatible?

According to Mark Thompson, timing can make or break your controlled impedance board. With many jobs going through turnkey environments, late communication about impedance issues takes valuable time out of the fabrication process and can delay delivery of product, leaving the end-user and the turnkey assembler unhappy.

This column is written from the viewpoint of you, the customer. What should you look for when qualifying a fabricator? Sure, you want the company to be IPC Class 3 6012 capable and ISO-certified, and you may need them to be ITAR-certified as well. But what other criteria can help you separate the wheat from the chaff, so to speak?

Keeping those impedance-controlled lines on the inside layers of a circuit board is a great idea for a number of reasons. Let's start with the facts: You'll make your fabricator and your customer very happy by remembering to keep them on the inside.

2009

A typical CAM department makes numerous edits prior to fabrication. Today, I will elaborate on inner-layer feature CAM edits, including the addition of flow and starburst patterns and constraints for scored jobs, as well as the process for fabricating edge-plated features.

When we talk about signal integrity or impedance lines, there are some very basic guidelines to follow. Remember, impedance mismatches cause signal reflections, which reduce voltage and timing margins.